Nondestructive eddy current technology is an established technology and various inspection techniques exist. Various types of eddy current probes and probe arrays exist. A probe array for an inspection system is typically comprised of a plurality of like probe elements. Most often the elements are traditional eddy current probes which are used in conventional mechanical scanning modes. Scanning is accomplished using typical probes operating in a bridge circuit or in reflection mode. Such probes utilize multi-turn induction coils often surrounding ferrite cores to intensify induced magnetic field flux. In reflection mode, one of the induction coils, the drive coil, is disposed very near the surface of a conductive part undergoing inspection and driven by an alternating current source to create a flux of magnetic field into and below the conductive surface. This flux causes local current to flow in the conductive part. This local current flow creates magnetic flux of its own. A complementary coil, the sense coil, operates to receive current mutually induced by the resultant flux due to current flow through the conductive part. Any flaw or defect in the near surface integrity of the conductive part will disrupt the flow of induced current. This disruption is detected as a change in voltage as detected by the sense coil.
A standard eddy current probe generally has coils disposed within close proximity of one another in order to responsively detect voltage changes induced by surface current disruptions. The probes may differ in their winding arrangement and coil connections. Coils may be wound in the same or opposite directions. The output is a differential voltage which may be used to produce an eddy current image. Response signals are collected from such probes by using manual or mechanical scanning modes. Scanning along the surface of the conductive part being inspected is typically accomplished by moving a single probe across the conductive surface to cover all regions of interest. This simple scanning measurement approach detects flaws by thresholding, i.e. using a pre-selected threshold to determine if a flaw signal is present. A primary problem with thresholding involves distinguishing a small disruptive flaw signal above background noise. The detection problem is complicated further as eddy current probes are themselves a source of great variability. In addition, the overwhelming relative size difference between the probe size and the flaw size causes spatial blurring in the resultant image.
Electromagnetic sensor arrays are a well established art; however, little has been done to apply the techniques employed for electromagnetic sensor array signal collection and image processing to eddy current nondestructive testing. It is recognized that scan rate efficiency is increased if probes are configured into surface measurement arrays so that large planar inspection surfaces can be scanned. Planar probe arrays typically employ like eddy current probes. Probe sensitivity to flaw detection is limited by the relative size of the probe sense element. It has been suggested that printed circuit technology be applied to fabricate more sensitive arrays by decreasing sense coil size. However, conventional printed circuit technology cannot achieve the miniaturization required to achieve current U.S. Government standards for flaw detection in difficult geometries like aircraft engines. Although it has been suggested that multi-layer printed circuit probe arrays could be implemented, there has been no suggestion that these arrays be driven using multi-frequency excitation, whether simultaneously mixed or independently applied.
Multi-frequency inspection techniques are not new. Multi-frequency techniques have been conventionally applied to achieve suppression of unwanted signals often due to the geometry of the surface under inspection. Simultaneous parallel frequency mixing or serial frequency mixing is conventionally applied to "blank" one or more unwanted signals. The multi-frequency driving technique has not been applied to an eddy current array probe, nor has frequency mixing of this sort been used to simultaneously "tune" flux penetration depth governing eddy current detection and flaw resolution sensitivity. Furthermore, the technique is not typically applied to a multiplicity of different probe elements. The operating frequency sensitivities of many different probe elements can be selected to provide specific depth penetration then drives can be switched among the probe elements to provide high and low resolution scanning. This provides a "selective" tuning capability. This novel capability relies on operational features of the eddy current inspection system described in patent application Ser. No. 07/699,456, entitled "Method and Apparatus for Nondestructive Surface Flaw Detection" by John D. Young et al. The notion of driving select multiple probe elements having unique characteristics with select multiple drive frequencies in order to tailor detection sensitivity is new.